That being said this all assumed that Russia had a strong air-defence. The various strikes that Ukraine has been able to make on Russian territory seems to have challenged that idea. I don't know how confident Putin feels he could block such attack, so that adds to the dissuasion too. The thing with nukes is that it only takes one missile to go through.
I don’t think France would ever strike first with nukes, even against Russia invading France itself. Conventional munitions and bombs are justifiable, but I can’t see France nuking any EU country that is being invaded by Russia or anyone else, even as a last resort. There’s nothing to gain militarily by doing so, and it would only give their enemies justification to escalate.
I honestly don’t know much about French nuclear doctrine and policy, so I would be happy to be corrected or pointed in the right direction.
Nuclear weapons as an ultimate warning is the doctrine: https://en.wikipedia.org/wiki/Air-sol_moyenne_port%C3%A9e
Whether it would be followed in practice is anyone's guess. But place yourself in the shoes of the invading country. You are basically facing the same question. Would France ultimately use nuclear weapons? You may think that they will likely not. But if they do, that's a path to having your cities and basically your civilisation wiped out. So at what percentage probability are you still happy to try your luck at invading France? Unless you are a Hitler or Stalin, who were happy to spend tens of millions of their own population without a second thought, there is no scenario where a rational leader will be taking that risk. And therefore deterence is achieved.
https://en.wikipedia.org/wiki/Maginot_Line